Not much of this makes any sense. These informations are invalid or outdated, though their recent date of creation.
The actual google sites differ heavily from your screenshots, e.g. Google only gave me the API-Key with no Client ID and Secret.
And when trying to register for OAuth it appears to be leading to a form where Developers are supposed to register their App using the API.
Is there no other way to use File Uploader for YouTube?
//Edit: I somehow made it through the whole process by going back and forth over and over. (The screenshots above still were not much like what was presented to me though...)
I was able to successfully add YouTube, but the accomplished upload speeds are devastating compared to what I was used to have with File Uploader. ~450 KByte/s instead of up to 7 MByte/s.